It has been demonstrated that the ambient noise field in the ocean over
continental shelf regions contains sufficient environmental information to
allow for inversion of the geoacoustic parameters of the seabed. In past
studies both compressional and shear wave speeds have been obtained which are
in good agreement with measurements made using independent methods. Other
factors which affect the coherence and influence the noise inversions for
basement parameters include the sea state and sound-speed profile. We have
observed that for low sea states, the ambient noise field contains little or no
contribution from the continuous spectrum. Theoretical studies, on the other
hand, indicate that the effect of the continuous spectrum is significant for
soft seabeds. One explanation for its absence is that at low sea states there
are few surface acoustic events close to the observation point, implying that
the standard statistical model used to calculate the coherence is not valid for
overhead sources. The density of surface acoustic sources in relation to the
spatial structure of the ambient noise field will be explored. [Work supported
by ONR.]